Alpha-lipoic acid as a pleiotropic compound with potential therapeutic use in diabetes and other chronic diseases

Insulin pathway

ALA has many actions in the insulin metabolic pathways, glucose uptake and glycogen synthesis with some differences between both isomers. In vitro studies have shown that R-ALA increases the translocation of GLUT1 and GLUT4 to the plasmatic membrane of adipocytes [21, 41–43] and skeletal muscle cells kept in a culture milieu [42]. Moreover, these events are associated with enhanced activity of proteins of the insulin signaling pathway like insulin receptor (IR), insulin receptor substrate 1 (IRS1), Phosphatidylinositide 3-kinase (PI3K) and protein kinase B (AKT) [42]. These events were also observed when Zucker rats, that are animal models of insulin resistance, were studied [43]. There is still a controversy if this action of ALA is totally insulin-dependent or not [41–43]. Furthermore, in an animal model of isolated rat muscles ALA inhibited glycogen synthesis, and action which is considered to have a pro-oxidant effect [32]. In general the S-ALA did not show a significant effect upon glucose disposal [41–43].

Nuclear factor kappa B

Nuclear factor kappa B (NFkB) is a transcription factor which is maintained in an inactive form in the cytosol because of its capacity in binding to an inhibitor kinase of NFkB activity, IKK [44]. Oxidative-stress is associated with hyperglycemia [45–48], and the existence of other conditions like viruses, some pathogens, and radiation are supposed to phosphorylate IkB resulting in its degradation, liberation and activation of NFkB which translocates to the nucleus to induce the transcription of several molecules related to inflammation, vascular adhesion and migration of monocytes [44]. ALA inhibits NFkB probably because of its action on the inhibition of the degradation of IkB through modulation of upstream kinases like MAPK [12] or its ability to regenerate vitamin E resulting in inhibition of protein kinase C which is also able to phosphorylate IkB [49]. This action of ALA seems to be independent of its antioxidant action [50]. Several experimental studies in vitro[50] and in vivo in rats [51] have shown the anti-inflammatory actions of ALA but few studies have addressed this subject in humans [52].

Adenosine monophosfatase protein kinase

ALA has some important functions in the activity and expression of 5' adenosine monophosphate-activated protein kinase (AMPK) in peripheral tissues and in brain (hypothalamus). AMPK is considered a multifunctional protein involved in many intracellular pathways related to metabolism, stress response, cell cycle and ageing [53]. Currently, AMPK plays an important role in linking nutritional factors and cancer and is considered to be a promising therapeutic target for cancer prevention and treatment [54].

AMPK is a cellular energy sensor and it is activated by liver kinase B1 (LKB-1) and Ca/calmodulin dependent protein kinase (CaMKK). Activation of AMPK results in down regulation of transcriptional events that promote synthesis of gluconeogenic enzymes, synthesis of fatty acids and up regulation of metabolic pathways resulting in an increased ATP production through glucose and fatty acids oxidation [54]. Moreover, AMPK can induce trasnlocation of GLUT4 to plasma membrane independent of insulin action [54].

So far, it is still unknown which is the mechanism that induces activation of AMPK by ALA in peripheral tissues. According to one study with myoblast cells culture, ALA could increase the activation of AMPK indirectly by activating CaMKK which becomes activated by an increase in intracellular Ca²⁺[55]. The inhibition of CaMKK abolishes this action [55]. Indeed, these actions of ALA-AMPK could also increase energy expenditure by increasing the activity of protein kinase-peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1-alpha) signaling pathway which is responsible for mitochondrial biogenesis [56]. All the above-mentioned actions resulting from ALA activating AMPK will cause a decrease in plasma glucose, an increase in insulin sensitivity and probably weight loss [53]. Recently it was described an action of ALA upon AMPKK in insulinoma cells culture and isolated islets of Langerhans which resulted in a decreased insulin secretion by 25 to 30% at low (2.8 mmol/L) and high (15 mmmol/L) glucose concentrations [57]. This event was observed both with acute and chronic treatment with ALA. Considering this latter action, another study showed a protective effect of ALA upon 2-deoxy-D-ribose induced oxidative damage and insulin expression in cultured cells and rat islets [58]. This protective action seems to be related to an increased intracellular GSH concentration. Both studies have brought to light important informations about the multiple effects of ALA upon beta cells. The study design, the stimulus and the concentration of ALA used in the experiments could have accounted for the different results that have been found [58].

ALA is also able to modulate the activity of AMPK in the brain by metabolic stresses that inhibit ATP production such as ischemia, hypoxia, glucose deprivation as well as by oxidative stress [59]. Recent studies of experimental neuronal models are pointing to AMPK’s possible roles beyond energy sensing, s ome reporting brain protective effects [60], while others reporting detrimental effects [61]. These discrepancies may be related to the dose of glucose added in the culture medium.

It has also been found that hypothalamic AMPK is important in the central regulation of appetite and energy expenditure and that ALA exerts anti-obesity effects by suppressing hypothalamic AMPK activity [62]. In this setting, ALA has an anorexic property and should be a potential anti-obesity drug [63].

Brain diseases and cognitive dysfunction

Brain is considered to be one of the major glucose and oxygen consumer organs, although it corresponds to only 2% of the total body weight [73]. Neurons and astrocytes are the most active cells of neurometabolism and are considered to be the neurometabolic couple. Currently, oxidative stress, an imbalance between the production of ROS and the antioxidant defenses plays an important function in the occurrence of neurodegenerative diseases as well in brain injuries, mediated mainly by mitochondrial dysfunction [14, 64, 74]. Moreover, the brain has a great sensitivity to oxidative stress-induced damage [75]. Considering ALA as an antioxidant compound its use has been reported in some brain diseases and associated with cognitive dysfunction such as Down syndrome [15] and AD [76, 77].

In a randomized, double blind placebo- controlled study in patients with Down Syndrome with pre-dementia, the use of 900 UI of alpha-tocopherol plus 200 mg of ascorbic acid and 600 mg of ALA for two years did not show improvement in cognitive dysfunction or stabilization of cognitive decline [15]. It is well known that these patients are at an increase risk in developing AD, and oxidative stress is considered to be an important feature of the Down syndrome.

In AD, ALA in association with n-acetylcysteine has shown to have a protective effect upon oxidative stress in fibroblasts decreasing caspase proteins which were responsible for apoptotic processes in patients with AD [14]. In a triple transgenic animal model of AD, ALA was able to improve neurons plasticity and improve many pathways of insulin signaling in the brain similar to the action described with metformin [77, 78]. So far, only two clinical studies have addressed the use of ALA (600 mg/daily) in patients with AD, both studies were open-labeled [79, 80]. One study was conducted with nine patients followed by 12 months [79] and the other with 43 patients followed by 24 months [68], both showing a slowing in the disease progression. Meanwhile, when ALA was associated with exercise training in animal model an increase in some antioxidant enzymes were observed [81].

Obesity

The increasing prevalence of obesity worldwide is an important epidemiological issue because it is occurring in parallel with the increase in the prevalence of DM and cardiovascular disease (CVD). Moreover, it is well known that both conditions are associated with insulin resistance, an increased plasmatic level of free fatty acids, of pro-inflammatory cytokines such as tumor necrosis factor alfa (TNF-α), interleukin 6 (IL-6) and decreased levels of adiponectin which is considered to be a protective cytokine [65, 82, 83]. The above-mentioned mechanisms seem to be related to oxidative stress and activation of NfKB [84]. ALA has many actions that may result in weight loss such as activation of AMPK in peripheral and brain tissue [59], inhibition of NfKB [44] and adipocyte differentiation [65]. Animal studies showed that rats fed with a high fat diet with ALA supplementation had less weight gain and better plasmatic lipid profile than the control group [65]. Some of these effects, such as the increase in HDL-cholesterol and the decrease in LDL-cholesterol levels were dose dependent. Some studies suggest that the ability of ALA supplementation in preventing insulin resistance might be related in part to the stimulation of AMPK and adiponectin in white adipose tissue [82] and attenuation of monocyte chemokine protein 1 (MCP-1) and TNF-α [71]. The authors suggested that ALA may modulate visceral adipose inflammation.

Data from human studies have shown conflicting results related to lipid metabolism [8, 85–88]. Considering weight loss, the studies have used doses of ALA ranging from 1,000 mg to 1,800 mg for up 20 weeks in obese patients with or without glucose intolerance and have shown a weight loss around 3 kg [8, 87, 88], that corresponds to a 3% of weight loss [87]. It is important to emphasize that the use of sibutramine for one year at a constant dose of 15 mg/daily resulted in a weight loss of about 7% [89]. However, in obese glucose intolerant subjects ALA has only shown an increase in LDL-oxidation in comparison to subjects who completed 12 weeks of ALA used associated with exercise [8]. Another study did not show any advantage of ALA supplementation for two weeks over lipid-induced insulin resistance in obese or overweight subjects [85]. However, an intravenous treatment with 600mg of ALA for two weeks in obese patients with glucose intolerance resulted in improvement of insulin resistance, decreased levels of free fatty acids, LDL-cholesterol as well oxidized LDL, TNF α and IL-6 [86].

The use of 1,200 mg/d of ALA for 10 weeks in an open-label pilot trial in patients with schizophrenia without DM taking atypical antipsychotic drugs showed a weight loss of -2.2 kg ± 2.5 kg. The weight loss was greater in patients taking antihistaminic antipsychotic agents, mainly clozapine, olanzapine or quetiapine[88].

Further studies are necessary to address the clinical use of ALA as anti-obesity drug with more complete data about dietary habits including the ingestion of fruits and vegetables which are the main source of antioxidants in a regular diet.

Nonalcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is considered the most prevalent liver disease worldwide. NAFLD is frequently associated with metabolic syndrome, obesity, DM and dyslipidemia [90]. During the natural history of NAFLD up to 25% of the patients developed non-alcoholic steatohepatites (NASH) [91]. Currently, mitochondria dysfunction, oxidative stress and inflammation play a key role in the pathogenesis of NAFLD and NASH [55]. Some animal studies have brought to light the possible mechanisms involved in the action of ALA in NAFLD and NASH in the last years [92, 93]. In one study, the use of ALA was followed by improvement in serum levels of insulin, free fatty acids, glucose, IL-6, triglycerides and markers of inflammation and of innate immune activation (Toll-like receptor 4, TLR4) in liver biopsy [93]. Two other studies using animals fed with a high fat diet showed that ALA induced an increase in uncoupling protein 2 which inhibits electron transport chain resulting in decreased ATP and lipid synthesis [92]. Moreover this action on mitochondria efficiency seems to be related to an increased action of the sirtuin proteins [94]. These proteins have many actions in several intracellular pathways associated with antioxidant defense [95, 96].

Burning mouth syndrome

Burning mouth syndrome (BMS) is a chronic disease characterized by pain, burning and itching of the oral cavity without association to any systemic disease. Sometimes xerostomia and dysgeusia could be present [97]. BMS is more prevalent in women in the menopause period. In Brazil its prevalence is of 1% [98]. The etiology is probably multifatorial including psychiatric diseases and hypothyroidism [99]. ALA was used during two months in patients with BMS in a dose of 600 mg daily but with conflicting results [98–100].

Cardiovascular disease and endothelial function

The main cause of mortality in non diabetic as well as in diabetic subjects worldwide is CVD [101]. CVD is multifactorial being the oxidative stress and a pro-inflammatory state considered to be the most important mechanisms involved in the large spectrum of CVD [102]. In this setting, ALA which has antioxidant as well anti-inflammatory actions has been used in several studies, both animal [8–10, 68–70] and human [9, 103] addressing different aspects of CVD.

For instance, the acute use of ALA in a 600 mg dose, associated with 1,000 mg of vitamin C and 600 IU of Vitamin E was able to ameliorate markers of oxidative stress and endothelial dysfunction evaluated by flow-mediated vasodilation (FMD) of the brachial artery in the elderly [7]. The effects of ALA upon endothelial function and markers of oxidative stress were age dependent and it was not observed in young subjects. In contrast, a review of many clinical trials using chronic antioxidant therapy was not able to demonstrate benefits on CVD [103].

In animal models, those fed with a high cholesterol diet, the use of ALA for 12 weeks reduced oxidative stress and weight and improved vascular reactivity [10]. Moreover, a reduction in the wall volume of abdominal aorta with slowing rate of the plaque progression and a reduction of the expression of adhesion molecules in thoracic aorta were also observed. One important finding in this study was the demonstration that ALA decreased the activation of NfKB which regulates the expression of pro-inflammatory genes as well adhesion molecules [10].

The effects and mechanisms of ALA on myocardial infarct size and diabetic cardiomyopathy which is defined as a ventricular dysfunction in diabetic patients without any other cause, were also evaluated in animal studies [66, 67]. Cardiac fibrosis which is the main feature of cardiomyopathy, was investigated in animal with streptozotocin (STZ) DM-induced [66]. In these animals the use of ALA had different actions such as improvement of cardiac function and cardiac fibrosis. Analyzing the left ventricular sections of these animals it was observed a better oxidative stress profile and a decreased expression of transforming growth factor β and smooth muscle actin, both associated with collagen production.

In animal models of ischemia-reperfusion it was demonstrated that ALA ameliorates cardiac dysfunction with a decrease in the infarct size, TNF-α, mieloperoxidase, markers of cell death (lactate dehydrogenase and creatinine kinase), and upregulates gene expression of several antioxidant enzymes [67]. The mechanisms of action of ALA were related to the phosphorylation of PI3K/AKT without any effect on nitric oxide production. Additionally, all these actions were dose dependent, being the dose of 15mg/kg the most effective. No effects with lower or higher doses were observed [67].

A benefit obtained with the use of ALA in hypertension was described and could be related to oxidative stress as well as to the modulation of intracellular Ca²⁺[3]. In animal studies of glucocorticoid-induced hypertension, the use of ALA prevented only dexamethasone induced-hypertension [65]. In human studies the use of ALA as a hypotensive agent presented conflicting results showing improvement or no effect [52, 104]. For instance, the Island Study which used ALA (300 mg/d) plus irbesartan (150 mg/d) for 4 weeks in subjects with metabolic syndrome did not find any difference in blood pressure albeit an improvement in flow mediated vasodilation and a reduction in plasma levels of IL-6 and 8-isoprostane [52]. Moreover it was also demonstrated that both drugs had a synergistic effect upon markers of endothelial dysfunction, inflammation and oxidative stress. It is important to emphasize that this study was not designed to evaluate blood pressure and the dose used of ALA was lower than the doses that are usually employed.

Cancer

Oxidative stress plays an important role in tumorigenesis [105]. ALA has been used as an anticancer agent mainly in experimental studies of different tumorigenesis cells type with promising results [16, 106–110]. So far the exact molecular mechanisms involved in this action are unknown. Besides its antioxidant acitivity, another possibility could be its relation to the capacity of inducing cellular apoptosis as recently demonstrated in lung cells [106]. This effects may result from activation of caspases proteins induced by endoplasmic reticulum stress [109]. Another hypothesis is associated with the metabolism of cancer cells which convert preferentially glucose to lactate, a mechanism known as the Warburg effect [108]. ALA is the cofactor of pyruvate deydrogenase which converts pyruvate to acetil CoA resulting in a decrease in the formation of lactate [107]. The net effect of this action is the inhibition of glycolysis. Additionally, an inhibition of mTOR (target of rapamycina), a signaling pathway responsible for cell growth and related to insulin receptor phosphorylation- PI3K-AKT activation, has been demonstrated in assays using insulinoma cells [57]. This action resulted in an inhibition of insulin secretion and of beta cells growth [57]. In contrast, recent data have demonstrated an antiapoptotic action of ALA due to a activation of PI3-K/AKT [111]. In addition, in this study it was also showed a direct binding site of ALA to insulin receptor [111]. It is possible to speculate that ALA can act in alternative routes resulting in different effects. The few studies in humans are case reports [111]. In these studies ALA was used associated with other antioxidant agents [111] or with other anticancer drugs [111].

Miscelaneous disorders

ALA has been used in other clinical conditions such as glaucoma [72] and osteoporosis [112, 113]. Both conditions are associated with an imbalance in the redox state. In a mouse model of glaucoma the increase in intraocular pressure was correlated to increased levels of lipid peroxidation and of oxidative stress-related genes expression in retina. Moreover, in these animals the addition of ALA to the diet enhanced antioxidant defenses, prevented retinal ganglion cell losses without significant intraocular pressure changes.

In a rat model of estrogen deficiency induced by ovariectomy the use of ALA increased bone mineral density (BMD) and decreased inflammatory markers such as TNF- α and IL-6. Besides these effects, the use of ALA also decreased the levels of osteopontin, a protein related to bone resorption [113]. In a model of low BMD induced by high-fat diet, which is a potent inducer of oxidative stress, the ALA supplementation resulted in an increase of the levels of expression of genes related to antioxidant enzymes, BMD, and biomarkers of bone formation, such as osteocalcin, and a down regulation of genes related to bone resorption activity, like osteoprotegerin, in femur biopsy. These studies indicated a possible action of ALA upon maintenance of bone balance.

Use of ALA in diabetes treatment

ALA has been used in 94 patients with T2M in a randomized, double-blind placebo-controlled study which has tested three antioxidants for three months in association with metformin and glimeperide: ALA, 300 mg/daily, ecosapentanoic acid, 180 mg plus docohexaenoic acid, 120 mg/daily [omega 3 fatty acids] and vitamin E 400 mg/daily [134]. Although an improvement in HbA1c, weight and waist have been observed with ALA, omega 3 fatty acids gave the better results concerning weight loss and glycemic control [134].

Use of ALA in diabetes-related chronic complications